1. Field of the Invention
The present invention relates to an optical diffusion film for use with liquid crystal displays, translucent rear projection screens and a process of producing the optical diffusion film.
2. Description of Related Art
There have been known various liquid crystal displays significantly improved in viewing angle characteristics on gradation and chromaticity such as disclosed in U.S. Pat. No. 2,378,252.
For the purpose of providing a brief background that will enhance an understanding of the present invention, reference is made to FIGS. 1 to 5.
Referring to FIGS. 1 and 2, the liquid crystal display 100 includes a liquid crystal panel 2, a backlight source 4 which illuminates the liquid crystal panel 2 with collimated light rays and an optical diffusion film 6 operative to diffuse light rays of an projected image spatially modulated by the liquid crystal panel 2. As schematically shown in FIG. 2, the optical diffusion film 6 comprises transparent microspheres or beads 14 uniformly distributed on a transparent substrate 10 and fixedly held between the transparent substrate 10 and a light absorbing layer 20. The transparent microspheres 14 are in close contact with and partly embedded in the transparent substrate 10.
Collimated light rays incoming the optical diffusion film 6 are refracted by the transparent microspheres 14 and come out of the optical diffusion film 6 passing through an interface between the transparent microspheres 14 and the transparent substrate 10. The light absorbing layer 20, except portions in contact with the transparent substrate 10, works as a black mask, so that the optical diffusion film 6 do not cause reflection and scatter of ambient light rays incident thereon from an viewing side. In this way the light rays incoming the optical diffusion film 6 are diffused. As a result, the optical diffusion film 6 cause the liquid crystal display 100 to display an image provide without a reduction in contrast. In addition the optical diffusion film 6 provides a satisfactory contrast over a wide range of viewing angle.
There have been widely utilized optical diffusion films that improve contrast of a projected image of the liquid crystal display 100. One of such the optical diffusion films that is practically used is shown in FIG. 3.
Referring to FIG. 3, an optical diffusion film 6 comprises a transparent base layer 12 formed over a transparent substrate 10, transparent microspheres 14 uniformly distributed and partly embedded in the transparent base layer 12 and a light absorbing layer 20 formed over the light absorbing layer 20 so as to have a pattern complementary to the distribution pattern of transparent microspheres 14. The optical diffusion film 6 is provided with the transparent base layer 12 in order to assure light transmissible areas 14a of the transparent microsphere 14 that are defined or masked by the light absorbing layer 20. In addition, the light absorbing layer 20 is formed on the transparent base layer 12 so as to absorb incoming light rays that are not part of a projected image and ambient light rays incident upon the light absorbing layer 20 from the viewing side. In consequence, the optical diffusion film 6 provides high light transmission efficiency and provide a projected image on the liquid crystal display 100 with satisfactory contrast over a wide range of viewing angle due to no reflection and scatter of light rays.
FIG. 4 schematically shows a process of producing the optical diffusion film 6 that is disclosed in, for example, Japanese Unexamined Patent Publication No. 9-318801. As shown in FIG. 3, the process comprises the steps of: preparing a transparent substrate 10 (step I); forming a transparent base layer 12 on the transparent substrate 10 and a light absorbing layer 20 over the transparent binder layer 12 (step II); closely and uniformly distributing a number of transparent microspheres 14 on the light absorbing layer 20 (step III); heating and pressing the layer of transparent microspheres 14 against the transparent binder layer 12 until the transparent microsphere 14 are partly embedded in the transparent binder layer 12 (step IV); and completing an optical diffusion film 6 by fixing the transparent microspheres 14 to the transparent binder layer 12 (step V).
The optical diffusion film 6 thus produced has the structure that the transparent microspheres 14 are masked or partly surrounded by the light absorbing layer 20 and embedded in the transparent binder layer 12. Due to this structure, the optical diffusion film 6 efficiently directs incoming light rays of a projected image to the transparent microspheres 14 closely distributed. Each of the transparent microsphere 14, that functions as a lens, converges the incoming light rays of the projected image and then diverges them, as a result of which the liquid crystal display 100 equipped with the optical diffusion film 6 is improved in the viewing angle characteristics. The incoming light rays into the optical diffusion film 6 that travels missing the transparent microspheres 14 are absorbed by the light absorbing layer 20 and, in consequence, do not come out of the optical diffusion film 6. Further, ambient light rays incident upon the optical diffusion film 6 from the viewing side are almost completely absorbed by the light absorbing layer 20, so as not to be observed as stray light. This causes the liquid crystal display 100 equipped with the optical diffusion film 6 to display an image with improved contrast.
However, as shown in FIG. 5, if the transparent microspheres are different in size, in other words, if there are transparent microspheres 142 and 143 smaller or larger in size than an average transparent microspheres 14, respectively, these transparent microspheres 14, 142 and 143 are not always distributed so as to be in uniform contact with the transparent substrate 10. That is, the smaller transparent microspheres 142 are possibly buried in the light absorbing layer 20 and isolated from the transparent substrate 10. As a result, the presence of smaller transparent microspheres 142 causes a reduction in transmittance of the optical diffusion film 6. On the other hand, the larger transparent microspheres 143 are possibly damaged when they are heated and pressed against the transparent binder layer 12. This leads to an occurrence of image defects due to absence of light.
As disclosed in, for example, Japanese Unexamined Patent Publication No. 11-102025, one of technical solutions to these problems of the optical diffusion film is to increase the transmittance of the optical diffusion film 6 by employing a transparent base layer having a melting viscosity n higher than a melting viscosity of light absorbing layer n′. According to the technical solution, because the light absorbing layer causes thermal deformations earlier than the transparent base layer during embedding the transparent microspheres in the transparent base layer, the transparent microspheres are easily embedded in the transparent base layer. This leads to an increase in transmittance of the optical diffusion film.
In the event where the pressure applied to the transparent microspheres is low and uniform during heating and pressing, there are some transparent microspheres that become isolated from the transparent base layer. This leads to an insufficient area of the transparent microsphere that is effective to transmit light rays and, as a result of which, the optical diffusion film is difficult to gain a sufficient increase in transmittance. This problem is significant when there are transparent microspheres having a wide range of sizes. On the other hand, in the event where the pressure applied to the transparent microspheres is too high, there occurs such damages of microspheres as to cause surface defects which result in absence of light.
Further, in recent years, there has been a strong demand of high performance of the optical diffusion film with the advance of high performance liquid crystal displays and projection screens. In particular, it is strongly called for that liquid crystal displays can display an image with high quality, i.e. high contrast and low coarseness. It is conceivably useful to employ microspheres small in particle size in order for the liquid crystal display to display an image free from coarseness. However, because it is essential to make the light absorbing layer thinner with a decrease in particle size of microspheres, the thickness of the light absorbing layer is possibly below a thickness necessary to provide a projected image with high contrast. In addition, a thickness of the transparent base layer is one of important factors decreasing contrast of a projected image. It is speculated that this decrease in contrast of a projected image is due to a ratio of an effective light transmissible surface area of a microsphere to a black-masked surface area of the microsphere and total internal reflection of ambient light rays by the microspheres. Accordingly it is conceivably effective to make the transparent base layer as thin as possible in order fort the liquid crystal display to provide a projected image with high contrast. In the event where the transparent base layer is too thin, the transparent base layer lowers its holding power for the microspheres and possibly allows the microspheres to come off therefrom during the step of forming the light absorbing layer. Further even though the transparent base layer has the lowest thickness necessary to securely hold the microspheres, it is hard for the liquid crystal display to prevent a decrease in contrast of a projected image.
In light of the drawback, it has been strong demand that the optical diffusion film comprises a transparent base layer having a sufficient thickness that does not lead a cause of a decrease in contrast of a projected image while it prevents coming off of microspheres.